Energy Transferred by Working
Electricity and Magnetism

How efficient is that?

Teaching Guidance for 14-16 Supporting Physics Teaching

Bringing precision to a term that is also used more widely

Wrong Track: Electrical transformers are very efficient because they work well and never break down.

Right Lines: Electrical transformers are very efficient devices because little energy is dissipated through heating. They are very efficient devices because all the power remains in the electrical working pathway: almost none is switched to other pathways.

Keeping a term for a particular use

Thinking about the teaching

The concept of efficiency has a precise meaning in physics, which is likely to be different from everyday understandings of what is meant by efficiency. The efficiency of any device is defined as: efficiency = energyoutenergyin × 100 %.

This relationship assumes that the energy output and input are measured over the same period of time. An equivalent form for the relationship is: efficiency = poweroutpowerin × 100 %.

Here we are comparing the energy out and in per second.

Supposing we think about a domestic filament bulb. For every 100 watt of power supplied to the bulb, about 5 watt goes to lighting the surroundings whilst the remaining 95 watt goes to heating the bulb and surroundings. The efficiency of the bulb can be calculated easily(using the relationship quoted above, efficiency = poweroutpowerin × 100 % and put the values in: 5 watt100 watt  ×  100 % to get an efficiency of 5 %.

In this case the powerout is taken as being the useful energy output per second. The job of an electric light bulb is to provide light. As can be seen from the calculation, it doesn't do this very efficiently because more power is switched to heating than to lighting.

Energy Transferred by Working
appears in the relation dU=dQ+dW
is used in analyses relating to Working Engines Thermionic Emission
is a special case of Work
has the special case Potential Energy Kinetic Energy
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